1. Field of the Invention
This invention relates to an eye fundus examining apparatus, for example, used to measure the shape of a blood vessel and a blood stream in the fundus portion of an eye.
2. Related Background Art
(1) An eye fundus blood stream meter is an apparatus that irradiates a blood vessel to be measured in the fundus of an eye to be examined with a laser beam, receives the scattered, reflected light thereof with a photodetector, detects the interference signal of a Doppler-shifted component, which is scattered reflected light from the blood vessel and scattered reflected light from a stationary blood vessel wall, frequency-analyzes this data and finds the velocity of the blood stream, the velocity of the blood stream (maximum velocity Vmax) being found from the following equation: EQU Vmax={.lambda./(n.multidot..alpha.)}.multidot..vertline..vertline..DELTA.fm ax1.vertline.-.vertline..DELTA.max2.vertline..vertline./cos.beta.(1)
where Afmax1 and Afmax2 are the maximum shifts of frequencies calculated from received light signals received by two light receiving devices, .lambda. is the wavelength of the laser, n is the refractive index of the measured region, .alpha. is the angle formed by two light receiving optical axes in the eye, and .beta. is the angle formed by a plane formed by the two light receiving optical axes in the eye and the velocity vector of the blood stream.
By effecting the measurement from two directions, the contribution in the direction of incidence of measurement light is offset, and the blood stream in any region on the fundus of the eye can be measured. Also, by making coincident the angle .beta. formed by the line of intersection between the plane formed by the two light receiving optical axes and the fundus of the eye and the velocity vector of the blood stream, .beta.=0.degree. is established and the true maximum blood stream velocity can be measured.
In this eye fundus blood stream meter utilizing a laser beam to measure the shape of the blood vessel and the velocity of the blood stream in a particular region of the blood vessel in the fundus portion of the eye, it is necessary for the measurement light beam to be accurately applied to the region to be measured within the measuring process time, but actually there is the fine movement or the like of the fixation of the eye to be examined and therefore, it is difficult to continue to apply the measurement light beam accurately to the region to be measured. Accordingly, apparatuses having tracking means for moving the applied position of the measurement light beam on the region to be measured at in real time corresponding to the fine movement of the fixation are disclosed in Japanese Patent Application Laid-Open No. 6-503733 and Japanese Patent Application Laid-Open No. 7-155299.
These apparatuses adopt a system in which tracking light from an illuminating light source in a tracking optical system and measurement light are applied to the fundus of an eye via a pupil-conjugate mirror lying at a position conjugate with the pupil, and the design of the apparatus is such that the spot of the measurement light is applied to a conjugate point on the fundus of the eye at a tracking reference position on a tracking sensor. A blood vessel is illuminated by the tracking light and is enlarged and projected onto a tracking sensor, and the pupil-conjugate mirror is moved so that the image of this blood vessel comes to the tracking reference position, whereby the measurement light always continues to irradiate a predetermined blood vessel.
(2) Also, when effecting measurement by the eye fundus blood stream meter, if the relative position of the optical system of the apparatus and a portion to be measured is changed by the fine movement or the like of the fixation of the eye to be examined, accurate measurement will become difficult and therefore, an apparatus which applies a light beam from a light source for tracking to a blood vessel to be measured, picks up the image of the blood vessel by a CCD camera, and scans the light beam from the light source for tracking so that the image of the blood vessel may be stabilized at a fixed position on the CCD camera, to thereby effect tracking, is disclosed in Japanese Patent Application Laid-Open No. 63-288133. In this apparatus, the reflected scattered light from the portion to be examined is very slight and therefore, light luminance is required of the light source for tracking and green light is suitable from the spectral absorbing characteristic of the fundus of the eye and a blood corpuscle and thus, an He-Ne laser source is used as the light source for tracking.
(a) In the above-described example (1) of the conventional art, however, there will be no problem if the point conjugate with the tracking reference position on the pupil-conjugate mirror and the measurement light spot are coincident with each other, but if the point conjugate with the tracking reference position on the pupil-conjugate mirror and the measurement light spot do not coincide with each other in terms of the structure of the apparatus and the principle of measurement, for example, for the reason when measurement beams are applied at a plurality of different angles, the positions of incidence of the tracking light and the measurement light differ from each other at the cornea position of the eye to be examined and thus, the center of the tracking light on the blood vessel and the measurement light do not coincide with each other. Also, when there is great corneal astigmatism or the like, a deviation occurs between the center of the tracking light on the blood vessel and the measurement light, and there arises the problem that accurate measurement cannot be accomplished in spite of the operating of the tracking system.
(b) Also, in the above-described example (2) of the conventional art, the quantity of scattered reflected light of the light beam from the light source for tracking near the blood vessel to be measured differs greatly depending on the state of the fundus of the eye to be examined or the region to be measured, and the quantity of light necessary to obtain the best image of the blood vessel also becomes different, for example, depending on the thickness of the blood vessel of the eye to be examined. In such a case, the contrast of the image of the blood vessel becomes low and tracking accuracy is reduced and the accuracy of the measurement of the blood stream velocity is also reduced.